RESULTS AND DISCUSSION
4.1 PERFORMANCE OF SEQUENTIAL ANAEROBIC–ANOXIC–AEROBIC CONTINUOUS MOVING BED REACTOR (CMBR) SYSTEM
4.1.4.2. Effect of varied influent ammonia concentration on anoxic CMBR (R2)
Steady state performance of R2 is shown in Tables 4.11 (a) and (b). In R2, influent NH4+– N concentration varied from 160 to 419 mg/L with respective loading of 0.107 to 0.279 g/L.day. Nearly 65–332 mg/L NH4+–N entered to R2 from as influent from effluent of R1 and recycle from R3 and some amount of NH4+–N was generated from SCN–degradation.
Accounting all this as total influent NH4+–N, R2 was capable of removing 31–12% NH4+– N and released 110–370 mg/L NH4+–N in the effluent [Figure 4.24 (a) and Figure 4.27 (a)]. NH4+–N removal rate in R2 was 0.031–0.033 g/L.day. NH4+–N removal in anoxic environment can occur through anoxic ammonium oxidation or assimilatory removal for biomass as widely reported (Caffaz et al. 2006; Jung et al. 2007; Fernández et al. 2010).
COD in influent of R2 increased from 2502–2710 mg/L with corresponding loading rate 1.67–1.80 g/L.day. The COD removal accounted in R2 decreased from 82% to 75% with increase in influent COD and NH4+–N showing COD removal rate of 1.267–1.383 g COD/L.day during the study. This comprised of 35–37% of total COD removal. Figure 4.26 (b) depicted that contribution of R2 in the system for COD removal was always higher compared to R1 and R3, suggesting insignificant inhibition of NH4+–N for present studied concentration on phenol/ or COD degradation in R2 [Figure 4.27 (a) and (b)].
Influent nitrate and nitrite concentration to R2 was 600–635 mg/L and 32–59 mg/L, respectively resulting in influent NOx––N loading to R2 0.421–0.462 g/L.day. The NOx––N concentration in the recycle increased as R3 released higher concentration of nitrate/nitrite
in effluent with increase in influent NH4+–N during the study. Respective NOx––N removal rate in R2 were observed as 0.295, 0.314, 0.334 and 0.375 g/L.day.
Figure 4.27 (a) Pollutant removal by R2 at varied NH4 +-N loading
50 60 70 80 90 100
0.00 0.05 0.10 0.15 0.20 0.25 0.30
NH4
+-N loading rate (g/L.day)
Removal (%)
0 5 10 15 20 25 30 35
NH4+ -N removal (%)
Thiocyanate Phenol COD NOx-N NH4+N
Figure 4.27 b) Pollutant removal rates in R2 at varied NH4 +-N loading
0.0 0.5 1.0 1.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 NH4
+-N loading (g/L.day) Pollutant removal rate (g/L.day)
0.00 0.01 0.02 0.03 0.04 0.05
Ammonia removal rate (g/L.day)
Phenol COD Thiocyanate
NOx-N Ammonia-N
Table 4.11 (a) shows that nitrite (NO2––N) was completely utilized in R2. However, denitrification was incomplete as nitrate was detected in effluent of R2 and anoxic condition in R2 existed through out the study. Figure 4.28 (a) shows that R2 was more
removal during the study as denitrification was the main route of nitrogen removal from the system.
Table 4.11 (a): Performance of anoxic CMBR (R2) at feed NH4+
–N variation
NH4+–N COD NO3––N NO2––N
S0A Se Rem S0 Se Rem S0 Se S0 Se Rem
COD:
Nrem
CODB TVS (mg/L) 160 110
(0)
31.2 2502 428 (11)
82.90 600 190 (7.0)
32 0 69.96 6.1 53 12090
(236) 268 220
(0)
17.9 2542 513 (34)
79.82 620 189 (0.9)
40 0 71.36 5.4 47 13050
(286) 368 322
(13)
12.4 2561 666 (10)
74.10 635 173 (8.6)
50 0 74.74 4.6 37 13629
(75) 419 370
(14)
11.7 2710 670 (23)
75.28 635 131 (9.8)
59 0 81.12 4.2 32 11296
(92) S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%),
A Influent NH4+–N of R2 = {Effluent NH4+–N of (R1+R3)/2 + 0.24x (SCN– removed in R2)}.
CODB: COD fraction for biomass (%)
Numbers in parenthesis indicate standard deviation values
Figure 4.28 (a) Fraction nitrogen removal by R2 and R3 with varied feed ammonia concentration
0 10 20 30 40 50
100 300 500 600
Feed NH4
+-N (mg/L) Fraction Nitrogen Removal (%)
R2 R3
COD in R2 got utilized for biomass synthesis and NOx––N reduction. COD/N removed ratio was calculated using equation 4.5. With increased influent NH4+–N and COD:N
removed ratio was 6.1–4.2. COD fraction available for biomass decreased from 53% to 32% with decreased COD removal and the observed yield of biomass decreased from 0.37 to 0.23 with increase in influent NH4+–N in R2. The biomass strength in R2 during the feed NH4+–N variation study is given in Table 4.11 (a). Total biomass concentration decreased to ~11000 mg/L at maximum influent NH4+–N, which was 12000–13000 mg/L in low feed concentration. Attached biomass in sponge cube was 9000–9800 mg/L. With increase in influent NH4+–N 160–368 mg/L, suspended biomass initially increased from 3000–3700 mg/L, however decreased to 2000 mg/L with further increase of influent NH4+–N to 419 mg/L. Attached biomass to suspended biomass ratio was 2.2–4.6 being higher towards higher influent NH4+–N concentration.
Table 4.11 (b): Performance of anoxic CMBR (R2) at feed NH4+
–N variation NH4+
–N
Phenol Thiocyanate SO4–2 pH
S0 S0 Se Rem S0 Se Rem S0 Se Gen Th.
SO42–
Err Se
160 409 2 (0) 99.51 400 2 (0.5)
99.50 475 938 (16.8)
463 657 –194 8.1
±0.2
268 416 5
(1.2)
98.80 401 5 (1.5)
98.75 465 920 (6.8)
455 652 –197 8.2
±0.2 368 468 5 (0) 98.93 401 10
(1.5)
97.57 454 888 (34)
434 644 –210 8.4±
0.2 419 551 15
(0)
97.28 401 40 (6.7)
90.02 465 880 (5)
415 596 –181 8.4±
0.2 S0: Influent (mg/L), Se: Effluent (mg/L), Rem: Removal (%), Gen: Generation (mg/L);
Th. SO42–: Theoretical sulfate generation {1.65*(SCN–removed)}, Err: Error (mg/L) Numbers in parenthesis indicate standard deviation values.
With increase in feed NH4+–N, influent phenol and COD to R2 increased as R1 released increased amount of phenol in its effluent correspondingly. Influent phenol to R2 was 409–
551 mg/L and corresponding phenol loading rate was 0.272–0.367 g/L.day. R2 satisfactorily removed 97–99% influent phenol and the contribution of R2 in total phenol
removal increased with increased influent phenol irrespective of ammonia loading. Phenol removal rate in R2 was 0.271–0.357 g/L.day. Eiroa et al. (2008) reported 91–90% phenol removal at phenol loading rate of 0.04–0.59 g/L.day in anoxic unit treating wastewater from resin producing industry containing 348–282 mg/L NH4+–N and NH4+–N removal in aerobic unit. In R2, influent pH was 7.5–7.7 and this increased to 8.1–8.4, which was due to denitrification.
SCN–in influent of R2 was ~ 401 mg/L with loading rate 0.267 g/L.day. SCN–degradation rate remained almost stable at 0.265– 0.261 g/L.day up to influent NH4+–N concentration 368 mg/L with SCN– removal 99–97%. SCN–degradation rate decreased to 0.241 g/L.day with 90% SCN–removal when influent NH4+–N concentration was 419 mg/L at loading of 0.279 g NH4+–N/L.day. The decrease might be associated with combine inhibitory effect of NH4+–N and phenol. In present study, while R2 was receiving maximum NH4+–N loading (0.279 g/L.day), it was accompanied with maximum phenol loading of 0.367 g/L.day and resulted in decrease in SCN– removal rate to 0.241 g/L.day and the removal efficiency decreased from 99% to 90%. Previous study with varied phenol HRT showed that phenol load more than 0.312 g/L.day caused decrease in thiocyanate removal in R2.
However R2 remained responsible for 45–49% of total SCN– removal with little decrease towards higher ammonia concentration [Figure 4.28 (b)]. R2 in present study sustained well to the toxic pollutants with better performance in terms of pollutant removal.
Figure 4.28 (b) Fraction thiocyanate removal by R2 and R3 with varied feed ammonia concentration
0 10 20 30 40 50 60
100 300 500 600
Feed NH4
+-N (mg/L) Fraction thiocyanate removal (%)
R2 R3
Figure 4.29 Error in sulfate generation against thiocyanate removal rate in R2
y = 951.82x - 47.138 R2 = 0.9739
0 50 100 150 200 250
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Thiocyanate removal rate (g/L.day) Error in sulfate generation (mg/L)
During the study, in R2, 415–460 mg/L sulfate evolved as byproduct from SCN– biodegradation however, sulfate generation was lower to the theoretical sulfate generation value throughout the study. Figure 4.29 shows increase in error of sulfate generation with thiocyanate removal rate in R2. During other studies with feed thiocyanate, HRT and feed phenol variation also the error was observed to be higher in R2 while R2 was showing higher thiocyanate removal rate. Other sulfur compound might have accumulated in R2 rather than sulfate/sulfide as earlier reported (Buisman et al. 1990; Mahmood et al. 2008).